Grounding and Bonding Part II

PART II

Last week we discussed a discovery where two panels, in a service for a newly converted gym, were not properly grounded. Article 250 is normally considered to cover two specific topics, Grounding and Bonding. However, it really covers three distinctly separate issues, and those are Grounding, "Earthing," and Bonding. Many in the industry are beginning to separate and treat the "Earthing" component as a majorly separate area. This helps to distinguish between grounding WITHIN a system FROM the grounding OF the system. Our issue here is more of an Earthing concern, where we must insure that stray currents and our "intentionally" grounded conductors have a safe and unfettered path to earth.


The initial 2008 NEC section to confer is Art. 250.52. This article explains what your accepted and physical grounding electrodes must be. Under "Electrodes Permitted for Grounding," there are 8 classes of Grounding Electrodes (GE) and 2 that are specifically prohibited. In most retro-fit remodel jobs, we would use two of these classes the most. These are our "Water Pipe" rule and "Ground Rods." We find the first of these two in section 250.52(A)(1) and the rods under 250.52(A)(5) "Rod and Pipe Electrodes."

There are a couple of installations rules to follow with these two types. The biggest of these is the so called "5-foot" rule. A water pipe that is FURTHER than 5 feet from the ENTRANCE of the building CANNOT be used as an acceptable GE. (Note, there is an exception to this; however, very FEW remodel jobs would ever meet the qualifications for this exception). We must also ensure that a minimum of 10' of continuously bonded metal pipe and casings exist that make DIRECT contact with earth for the entire minimum length. Water meters and the like must be jumpered over with a bonding conductor. Under Article 250.53 we also find a combining rule that REQUIRES an additional supplemental GE be used when we utilize the water piping system as a GE. Thus we see that we're required to use a ground rod or something similar in conjunction with our water pipe.


Ground rods must be at least 8' in length, and according to 250.53, 8' of that length MUST be in contact with soil. Most ground rods are manufactured in 10' lengths so that the bonding connector would not have to be buried in soil. Clamps that are buried must be listed for direct burial. The rod must be driven vertically, unless rock bottom is encountered. (In the Austin area, that is an extremely common situation. In that case, a 45 degree drive/burial angle is allowed or it may be completely buried laying flat, under a minimum of 30" of soil. DO NOT bend your ground rod into a 90 degree "stub up" configuration, where the top portion is above ground and the remainder is horizontally flat. This is a poor practice that is not only a code violation but is also a functionality liability! A situation that I've see and run across too many times beforehand)

There are two final rules to consider. The first is that you are not required to use a bonding jumper for the supplemental GE rod that is larger than a #6 AWG CU conductor. The second is that your resistance reading between the rod and earth shall be 25 ohms or less. The use of special grounding meters should be utilized to determine this value. Where it is greater than 25 ohms, additional rods should be installed at a distance NOT LESS than 6 feet apart. (See Art. 250.56).

Next week we will talk about how to apply Table 250.66 and review more installations rules and requirements. If you have specific questions about any grounding or bonding issues, we encourage you to send us an email. We will respond promptly and your question may even be featured on our web site!

Wire Ampacity and Conduit Fill Calculation

Wire Ampacity and Conduit Fill Calculation

This week I want to take a minute to review our wire ampacity and conduit fill (wire fill) calculation.

There seems to be a lot of common confusion out there when it comes to wire fill. Annex "C" in the back of the 2208 NEC © is a good tool, but it can be very dangerous to use it without considering the impact of de-rating factors found in Chapter 3.

We normally use a 20 amp circuit breaker to protect #12 AWG cu wire. Annex C tables show that we can safely place a total of 16 of this size (Type THHN, THWN, or THWN-2) in a 3/4" EMT conduit. this is only for the protection of the wire from a physical stand point. Annex C is only concerned with the protection of the wire during installation. It does NOT take the ampacity of the wire into consideration!

When de-rating wire, we begin with table 310.16 (page 147, 2008 NEC ©) and de-rate for our ambient temperature AS WELL AS the number of current carrying conductors. The de-rating table T310.15(b)(2)(a) on page 145 shows us:

"Adjustment Factors for more than Three Current carrying conductors in a Raceway or Cable"

Number of C.C.C.	% of 310.16 Values
4-6 Conductors	80%
7-9 Conductors	70%
10-20 Conductors	50%
21-30 Conductors	45%
31-40 Conductors	40%
41+ Conductors	35%

You can quickly see that in our earlier example, if we put 16, #12 THHN conductors in that 3/4" conduit, we would have to drop the circuit breaker to a 15 ampere size! Now seriously, how many electricians in the field really do that? You understand now how dangerous the casual use of Annex "C" can be.

For all practical purposes, only 9 current carrying conductors could be placed in that conduit without having to drop the OCPD (circuit breaker) size: [T310.16 value for #12 THHW=25 Amps. 25 Amps X 70% (the de-rating for 7-9 conductors) would equal 17.5 Amps, and due to 240.3(B) we are allowed to round up to a 20A breaker.]

The grounded conductor (neutral) can also cause confusion sometimes. It counts as a "current carrying conductor" in some cases, but not in others. If it is shared between two different phases in a multi-wire circuit, it usually does not count as a current carrier , as it only carries the unbalanced load in the circuit. [For example, a "full boat with circuits #3, #5, #7 in a 30 panel, would not usually have their neutral counted.]

Where non-linear loads are supplied however the neutrals DO count. If you have electronic fluorescent ballasts or heavy computer loads (etc...) on a shared branch circuit neutral, it will have harmonic distortions that will disrupt the cancellation of loads between phases and can actually cause larger loads on a neutral that are found on any single ungrounded phase conductor in the circuit.

If the (neutral) grounded conductor is NOT shared between phases, i.e. a single 120V single phase branch circuit, such as a general purpose receptacle outlet circuit conductor, it counts as a current carrying conductor. In this installation it carries the full load on it back to the distribution point. Two other common issues with Annex C are considerations for different sized EGC/GEC conductors, and differences between Annex C values and manual fill calculations using Chapter 9, tables 4 and 5.

The value of Annex C is a "staff it full" value. Therefore you would have to keep in mind your additional ground wire, which is usually a smaller size than that of the phase conductors. The only way to use Annex C in this case is to always consider your EGC or GEC as a full size conductor count. A manual calculation is more accurate than Annex C and it has been this author's observation that Annex C values sometimes vary quite a bit from a manually calculated fill based on Chapter 9 tables.

All of that considered, going back to our 3/4" raceway, we could put 3 "full boats" in this raceway (3 neutrals not counted, 9 phase conductors, and 1 equipment ground) for a total of 13 conductors. This results in 3 less than our "fill amount" according to our Annex C tables. Thus, our advice is to always carefully consider your ampacity de-rating factors as required by 310.15.

(Author's Note: As evidenced by the above, an electrician would have to refer to several different tables in order to correctly and safely calculate the correct fill and ampacity values for watch and every pipe run. we are excited to provide and industry first for all our members. Electrician Testing has painstakingly prepared a custom table; available only through us, that considers ALL required factors. Everything is brought together in one quick and easy to follow chart. Contact us today to get yours, and look for our upcoming publication that will include this chart along with many, many more custom time saving tables!)